For 300 bucks, I think you can only get those very low end Casio keyboards (those with built-in songs).. which I think is not worth... The entry level Yamaha portable keyboard with their very realistic grand piano (touch sensitive) sound is about 800 bucks..

Midi keyboards and portable keyboards are basically the same thing.. MIDI is a digital interface for audio triggering...Both these keyboards produce sound using synthesizers and samples.. You can try and find MIDI keyboard controllers..It looks like a keyboard which needs to be plugged into a PC that relies on the computer for sounds and effects... These are cheaper then workstations yet they are more expandable soundwise as all the sampling and processing are carried out in the PC..